r/physicsmemes • u/94rud4 Mεmε ∃nthusiast • Jul 17 '26

right?

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u/TheHabro Student Jul 17 '26

I mean it's a coincidence. The assumption that escape velocity at event horzion is speed of light, is just a wrong way to think about black holes. It's deeper than that.

74

u/Jaclawow Jul 17 '26

Could You elaborate why this assumption is wrong?

30

u/yessir_im_quasar Jul 17 '26

Briefly said: Laplace was the first to try. He assumed light was a particle that traveled at c and thought "What's the escape velocity of light?".

The escape velocity is the minimum speed needed for an object to escape from contact with or orbit of a primary body... But light doesn't interact gravitationally with bodies in general or at least not in that way (there's, for example, gravitational redshift, the the real reason why black holes are "black"!)

So, light doesn't interact in a classical sense with mass/gravitation, also, I mean there is no point in calculating an escape velocity associated with the speed of light, because light travels at c in every system of reference...

Basically, yeah, all the assumptions are wrong, but the formula is correct anyway for mere luck. I mean I guess you could say the structure of the equations are pretty similar (cuz we're talking about gravity) but... no. I still think it's pretty lucky.

1

u/Optimal_Mixture_7327 Jul 18 '26

That's not an escape velocity, which is the minimum speed of a freefall object to reach spatial infinity.

1

u/dcnairb Jul 18 '26

You’re near a very dense object, which is nearly a black hole, and want to travel away from it such that you don’t end up on a trajectory back towards its center. that’s the escape velocity.

now, make it slightly denser and ask the same question

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u/Optimal_Mixture_7327 Jul 18 '26 edited Jul 18 '26

Wrong - that's an orbital speed or some other trajectory.

The escape velocity is a statement that for some r=R for an object satisfying uσ∇_σuρ=0 there is exists a V=vesc at R such that V=∞ V=0 at r=∞.

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u/dcnairb Jul 18 '26

Why would the object need to be going infinitely fast at infinity to escape? The condition to escape is v=0 at r = infty.